As a conventional voltage-controlled oscillator, there is one that has an active element for oscillation composed of a field-effect transistor; a first reactance circuit connected to the source terminal of the field-effect transistor; a second reactance circuit connected to the gate terminal of the field-effect transistor; a third reactance circuit connected to the drain terminal of the field-effect transistor; a load resistance connected to the third reactance circuit for outputting the oscillation power amplified by the field-effect transistor; and a variable-capacitance element provided to the first reactance circuit for varying the capacitance in response to a control voltage as a tuned circuit for controlling the oscillation frequency.
The operation is as follow: Noise within the voltage-controlled oscillator circuit is amplified by the active element for oscillation, part of the amplified power is fed back to the active element for oscillation through the first to third reactance circuits connected to the active element for oscillation, and the active element for oscillation further amplifies the power, thereby carrying out the oscillation operation and causing the oscillation output to be produced from the load resistance. The oscillation frequency is determined by the resonance frequency of the tuned circuit. To control the oscillation frequency, the junction capacitance of the variable-capacitance element is varied by varying the control voltage applied to the variable-capacitance element, thereby varying the resonance frequency of the tuned circuit. Thus, the oscillation frequency varies. The relation between the oscillation frequency and the junction capacitance of the variable-capacitance element is given by the following expression.(fmax/fmin)2∝Cj-max/Cj-min where fmax and fmin are a maximum oscillation frequency and minimum oscillation frequency, respectively, and Cj-max and Cj-min are a maximum variable-capacitance value and minimum variable-capacitance value (for example, see Patent Document 1).
Patent Document 1: Japanese patent application laid-open No. 8-335828/1996.
With the foregoing configuration, the conventional voltage-controlled oscillator achieves a broad oscillation frequency band by greatly varying the junction capacitance of the variable-capacitance element. The conventional voltage-controlled oscillator, however, has a problem in that its oscillation frequency band is limited by a fixed capacitance in the active element for oscillation or in the reactance circuit, and the variation ratio of the junction capacitance of the variable-capacitance element.
The present invention is implemented to solve the foregoing problem. Therefore it is an object of the present invention to provide a voltage-controlled oscillator capable of broadening the oscillation frequency band by equivalently increasing the variation ratio of the capacitance of a combined variable-capacitance element.